An inverse correlation between cancer mortality rates and regional solar UV-B radiation exposure, a major source of vitamin D (vit D), has been found for cancers of the breast, colon, rectum, ovary, prostate, stomach, and numerous other cancers. Dietary and serum studies also demonstrate that vit D is a risk factor in colonic adenomas and for numerous cancers. The anticancer effect of vit D is strongly supported by in vitro and animal studies. However, no randomized clinical trials have used a vit D intervention sufficient to raise serum 25OHD to optimum levels and have targeted a cancer outcome. We reported a double-blind, placebo-controlled, randomized trial of calcium (Ca) and vit D supplementation in a population-based sample of postmenopausal women. A secondary objective of that study was to determine the efficacy of Ca alone and of Ca plus vit D3 in reducing risk of cancer of all types. We found that vit D3 significantly reduced the risk of all-types cancer by 60%. These findings need to be confirmed with a clinical trial designed with incidence of cancer as the primary outcome. In this application we propose to test whether increasing serum 25OHD to optimal levels, while maintaining adequate Ca intake, reduces the incidence of cancer in a population-based sample of postmenopausal women 60+ years of age. The Specific Aims are to: 1. Sample randomly the population of healthy independently-living postmenopausal women 60 years and older from 9 rural counties; 2. Enroll 2300 women into an intervention study, assign them randomly to 1 of 2 treatment groups: 1) vit D3 (2000 IU/d) and Ca (1200 mg/d), or 2) vit D3 placebo and Ca placebo, and to follow each subject for 4 years; 3. Collect and store white blood cells from every subject to test for genetic markers should the intervention be found effective; 4. Determine the effect of supplementation with vit D3 on incidence of all types of cancer combined; 5. Determine in a nested-case control study the association of serum 25OHD collected at baseline and the end of year 1 with risk of cancer over 4 years; 6. Determine the effect of supplementation with calcium and vit D3 on incidence of specific cancers: breast, lung, colon and myeloma, leukemia, lymphoma. 7. Determine the effect of supplementation on incidence of other disorders, specifically hypertension, cardiovascular disease, osteoarthritis, colonic adenomas and diabetes, upper respiratory infections and falls. At each 6-mo visit, we will assess: medical and social history; adverse events; cancer diagnoses; and adherence to supplement/placebo. Annually, we will obtain height and weight and analyze serum 25OHD. At baseline and end of 4 yrs, we will assess dietary intake and physical activity. The proposed study addresses the goal of NCI's Strategic Plan to eliminate the suffering and death due to cancer by 2015. It specifically addresses Strategic Objective 2, to accelerate progress in cancer prevention. Positive findings from our nutritional intervention study will result in an inexpensive, safe method of preventing cancer.